Dietary proteins influence body weight by affecting four targets for body weight regulation: satiety, thermogenesis, energy efficiency, and body composition. Protein ingestion results in higher ratings of satiety than equicaloric amounts of carbohydrates or fat. Their effect on satiety is mainly due to oxidation of amino acids fed in excess; this effect is higher with ingestion of specific "incomplete" proteins (vegetal) than with animal proteins. Diet-induced thermogenesis is higher for proteins than for other macronutrients. The increase in energy expenditure is caused by protein and urea synthesis and by gluconeogenesis. This effect is higher with animal proteins containing larger amounts of essential amino acids than with vegetable proteins. Specifically, diet-induced thermogenesis increases after protein ingestion by 20 - 30 %, but by only 5 - 10 % after carbohydrates and 0 - 5 % after ingestion of fat. Consumption of higher amounts of protein during dietary treatment of obesity resulted in greater weight loss than with lower amounts of protein in dietary studies lasting up to one year. During weight loss and decreased caloric intake, a relatively increased protein content of the diet maintained fat-free mass (i. e. muscle mass) and increased calcium balance, resulting in preservation of bone mineral content. This is of particular importance during weight loss after bariatric surgery because these patients are at risk for protein malnutrition. Adequate dietary protein intake in diabetes type 2 is of specific importance since proteins are relatively neutral with regard to glucose and lipid metabolism, and they preserve muscle and bone mass, which may be decreased in subjects with poorly controlled diabetes. Ingestion of dietary proteins in diabetes type 1 exerts a delayed postprandial increase in blood glucose levels due to protein-induced stimulation of pancreatic glucagon secretion. Higher than minimal amounts of protein in the diet needed for nitrogen balance may play an important role for the increasing number of elderly obese subjects in our industrialized societies, since proteins exert beneficial effects in the conditions of overweight, metabolic syndrome, cardiovascular risk factors, bone health, and sarcopenia. Adverse effects of increased dietary proteins have been observed in subjects with renal impairment- this problem is frequently observed in the elderly, hypertensive, and diabetic population. Nevertheless, dietary proteins deserve more attention than they have received in the past.